HELGOL~'qDER MEERESUNTERSUCHUNGEN Helgol~nder Meeresuntersuchungen 35, 329-339 (1982) Entry of migrating American eel leptocephali into the Gulf Stream system R. C. Kleckner & J. D. McCleave Migratory Fish Research Institute and Department of Zoology, University of Maine; Orono, Maine 04469, USA ABSTRACT: Abundance, size and depth distributions of American eel (Anguilla rostrata) lepto- cephali collected in four transects across the Florida Current between the Straits of Florida and Cape Hatteras, North Carolina, were examined in order to assess the relative importance of two migration routes into the Gulf Stream system. A nine-fold increase in the abundance and an increase in the mean length of leptocephali collected in the Florida Current north of the Straits of Florida indicate that most leptocephali enter the Gulf Stream system directly from the Sargasso Sea rather than by a more southern route through the Bahama Islands. Leptocephali were concentrated in the upper 140 m at night and upper 350 m during the day. Wide vertical ranges in daytime collections precluded more refined estimation of vertical distribution. The collection of eight small leptocephali (11 to 17 mm total length) in the western Caribbean Sea is discussed in relation to the origin of leptocephali taken in the Straits of Florida. INTRODUCTION Information on the migration routes followed by American eel, Anguilla rostrata, leptocephali between their natal area east of the Bahama Islands and the coast of North America is fragmentary and incomplete. Surface currents are no doubt responsible for the initial movement of the smallest larvae from the spawning area (Schmidt, 1922, 1925). Harden Jones (1968) speculated that surface currents in this region would carry American eel leptocephali north of the Bahama Island chain and into the Gulf Stream system east of Cape Hatteras, North Carolina. American eel leptocephali are common in summer collections along the main axis of the Gulf Stream system both north and south of Cape Hatteras (Vladykov & March, 1975). Leptocephali are also present in the Florida Current east of Miami, Florida, between May and August (Smith, 1968). It is not known whether these leptocephali enter the Florida Current from the east via the Old Bahama, Nicholas and Santaren Channels or from the Caribbean Sea via the Yucatan Strait and Gulf Loop Current. The studies cited above suggest a dual input of American eel leptocephali into the Gulf Stream system, one via the Florida Current and the other from the Sargasso Sea north of Grand Bahama Island. In order to assess the relative importance of the two migration routes, we have examined (1) the abundance, size and depth distributions of leptocephali collected along four transects crossing the main axis of the Florida Current between West Palm Beach, Florida, and Cape Hatteras, North Carolina, and (2) the © Biologische Anstalt Helgoland 0174-3597/82/0035/0329/$ 02.00 330 R.C. Kleckner & J, D. McCleave specimen and collection data for a group of small American eel leptocephali from the Yucatan Strait in the western Caribbean Sea, a potential source of leptocephali for the Florida Current. MATERIALS AND METHODS The Florida Current samples were borrowed from the Woods Hole Oceanographic Institution collections held by the Department of Fishes, Museum of Comparative Zoology, Harvard University, Cambridge, Massachusetts. They were collected between 29 July and 16 August 1978, on R.V. Oceanus cruise 49 by R. H. Backus and J. D. Craddock. Four trawl station transects were made perpendicular to the historical main 80 ° 75 ° 55 ° ee ° h~ Cape 90% Romain ". :% %,, 80 St Augustine 50 ° 96°e% • • • ~8 West Palm Beach , 103 ; ~, 25 ° Fig. 1. North Atlantic Ocean from Cape Hatteras to the Straits of Florida showing Florida Current transect and sample locations including station numbers axis of the Florida Current between West Palm Beach, Florida, and Cape Hatteras, North Carolina (Fig. 1). The transects extend seaward from slope water, across the Florida Current and into water of Sargasso Sea origin. All collections were taken with a 10 m Multiple Opening Closing Net and Environ- mental Sensing System (MOCNESS-10). This system fished a series of nets sequentially during each oblique trawl. The mouth area of the MOCNESS-10 was 10 m 2 when the frame of the net was tilted to 45 ° during fishing. The netting was 3.5 mm circular mesh throughout. Sensors on the MOCNESS-10 measured pressure {depth}, speed through the water, frame angle and net opening and closing. This information was transmitted via Entry of leptocephali into the Gulf Stream 331 the towing cable to shipboard displays and recorders. Estimates of the volume of water filtered by each net were calculated from this data. Identification of American and European eel leptocephali (A. rostrata and A. anguilla) followed Smith (1979). Total length was measured from the anterior tip of the teeth to the tip of the caudal fin rays. Total length is approximately 2 to 3 % longer than standard length, which is measured from the tip of the snout to the caudal fin base (F.-W. Tesch, personal communication). Specimens were fixed and preserved in 10 % formalin. Crude estimates of the relative abundance of American eel leptocephali at each trawl station were calculated as follows: (1) only nets with all or part of their sampling effort in the upper 140 m are included in the estimates (see "Results" for vertical distribution range), (2) estimates of the water volume filtered for nets including a sampling component below 140 m were proportionally reduced to include only the percentage filtered within 140 m of the surface, and (3) the total number of leptocephali collected by all trawls at a station were divided by the total estimated water volume filtered in the upper 140 m by all nets fished at the station. Temperature-depth data from expendable bathythermograph casts and characteri- zations of the associated mesopelagic fish fauna provided by J.D. Craddock were utilized to define the water mass sampled at each station. The Sargasso Sea fauna was characterized by the presence of Pollichthys mauli (Gonostomatidae) and Lepidophanes gaussi (Myctophidae), the Gulf Stream system fauna by tropical species, such as Lepidophanes guentheri, Diaphus dumerillii, and D. luetkeni (Myctophidae), and the slope water fauna by temperate species, such as Ceratoscopelus maderensis and Ben- thosema glaciale (Backus et al., 1970, 1977). For analyses of length and relative abundance, samples of leptocephali were grouped within transects by their associated types of mesopelagic fish fauna. Those samples whose fauna included slope water fauna or Sargasso Sea fauna as well as Gulf Stream fauna were grouped with samples whose fauna was a pure Gulf Stream one. A two-way analysis of variance of mean length by transect and faunal assemblage type was conducted using the general linear model for unbalanced data. Because the interaction between transect and faunal type was significant (P < 0.001), the following one-way analyses of variance were done: length by transect for Gulf Stream faunal type only, length by transect for Sargasso Sea faunal type only, and length by water type for each of the three northern transects. Duncan's test was applied if there were more than two levels of the independent variable. Significance criterion was P < 0.05. Similar analyses were not performed on the relative abundances, because the estimates of abundance are crude. Specimen and collection data for the Caribbean Sea samples were provided by D. G. Smith of the University of Texas Marine Biomedical Institute, Galveston, Texas. These samples were collected with a two-meter ring net of 570 ~m Nitex mesh at 20 ° 30' N, 87 ° 00' W between the Yucatan Peninsula and Isla Cozumel. RESULTS American eel leptocephali were concentrated in the upper 140 m at night in the transect collections. Ninety-six percent of all specimens taken in nets sampling discrete depth ranges at night were collected in this layer (Table i). Unfortunately, the narrowest 332 R.C. Kleckner & J. D. McCleave Table 1. Vertical distribution of American eel leptocephali in MOCNESS-10 collections in the Florida Current Depth range Number of Volume of water Catch rate (m) American eel filtered (No./104 m 3) leptocephali (104 m 3) Day 0- 350 65 (91.5 %) 36.8 1.77 340- 750 5 (7.0 %) 89.6 0.06 670-1200 1 (1.4 %) 52.4 0.02 Night 0- 140 340 (95.8 %) 67.5 5.04 90- 400 12 (3.3 %) 18,8 0.64 140-1000 2 (0.6 %) 67.9 0.03 depth range fished in the surface waters during the day was 0 to 350 m. Nets fished obliquely between 0 and 350 m collected 92 % of the specimens taken in daytime collections (Table 1). Clearly, American eel leptocephali in these collections did not migrate diurnally to depths below 350 m. The question of diel vertical migration within the upper 350 m is not resolved by these data. The mean total length of American eel leptocephali in collections associated with Gulf Stream mesopelagic fish fauna increased from south to north (Fig. 2). Cape Hatteras leptocephali (samples 115-118) were significantly longer than Cape Romain lepto- cephali (samples 85-87) which were in turn significantly longer than St. Augustine and West Palm Beach leptocephali (samples 91-94 and 99-106, respectively). Similarly the mean length of leptocephali associated with Sargasso Sea fauna was significantly greater at Cape Hatteras (samples 119-123) than at Cape Romain and St. Augustine (samples 80-83 and 97-98, respectively). 6- 5- 5- 51J- 29- 12 t5- 14- 8A 4A 6A IA 51J A I4A 15A I I I I I I 1 I I05 54- --I-- 52- E E - .=50-v 112 - 95 104 -{- 5O ~48- ÷ -I- t8 - 5 -~46- 25 I-- - 44- -I- GS GS SS GS SS SL GS SS i West 9'~- 9'~ 8'5- B'o- l lz- ris- ib- Palm 94 98 87 83 114 1t8 125 Beech St, Augustine Cape Romain Cape Hatteras Fig.